[IMP] web_editor: enable click on style tab

The goal of this commit is to enable the click on Style tab
in "Edit mode".
The click on this tab with no block selected will display a
message to inform the user that he has to select a block
before editing.

task-2443297

closes odoo/odoo#65573

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>